Piston pins too short?

[vandor]

Why do I always get the weird problems? I was assembling a customer's Fiat TC engine, when I noticed that the pistons pins that come with the new pistons are 5-6mm shorter than the space allotted to them, ie. the distance between the circlips.
I checked the stock Fiat pin, and it's about 1mm shorter than the distance between the circlips.
Now this looks very wrong, but then I know of engines that use teflon buttons to keep the wrist pin in place, and those likely do not fit very tight, so the pin may float side-to-side.
The short pin may not distribute the load evenly over the piston (possibly a non-issue), but would it be wrong to use the Fiat pins? I mainly worry about the piston pin clips coming out because they are not resting against the pin.
Thanks,

Csaba
Auto Ricambi, LLC
GC Book #288

[Guy Croft]

Yes, that clearance is way too big and would lead to the pins going thru the block after a while but c 1mm or less is OK. There is no virtue at all to having looser. Sounds like the pins have been swapped - Mondial forged slipper pistons (eg) had very short pins.

I assume these pistons have round wire clips? You can change to another pin set provided they have chamfered ends which you must use with wire clips (flat spiral wound clips need flat-faced pins) and of course provided they fit the pistons and pins.

You mention teflon bushes - they really don't last long - they wear in the vertical plane because of the hammering they get against the pin bore as they fly up and down: they should only be used on engines that get stripped very frequently and regularly inspected. I think they are a waste of time frankly and yes I have tried them (and caught them just in time..).

For those without Csaba's experience I will also point out that you should never use clips and bushes together as the whole thing will self-destruct.

I have no idea what the mechanism is that can force the pins sideways (never stopped to think about it I guess) but it does exist and the bigger the clearance the more damage can occur.

G

[vandor]

Hi Guy,

>I assume these pistons have round wire clips?

No, they have simple circlips, the type with two holes in the end, so one can use clip-pliers on them.
They are not nearly as heavy as the stock clips, ie. have much less of a spring rate.
bye,

Csaba

[Guy Croft]

You need to be careful then.

A circlip must not be used in a groove of semicircular form - it will come out. And - supposing the groove is of correct form for a circlip the pin would have to be flat faced as a chamfered pin would impose too much contact stress on the clip (due to the reduction in contact area).

G

[vandor]

Guy,

Thanks, I will check the shape of the groove and the pin. This all came as a package, but I no longer assume that a manufacturer knows what they are doing.
bye,

Csaba